1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device with a large screen having wide viewing angle characteristics.
2. Description of the Related Art
In the conventional technology, there has been a display device utilizing electro-optical effects, such as a twisted nematic (TN) or super twisted nematic (STN) type liquid crystal display device. Now technology has been vigorously studied such that the viewing angle of such a liquid crystal display device may be enhanced.
An example of the technology for enhancing the viewing angle is seen in a TN type liquid crystal display device in which liquid crystal molecules are axially symmetrically aligned in each of liquid crystal regions separated by a wall of polymer (protrusion-like structure), namely, a so-called Axially Symmetrically aligned Microcell (ASM) mode liquid crystal display device, as disclosed in Japanese Laid-Open Publication No. 7-120728. A liquid crystal region surrounded by the polymer wall typically corresponds to a pixel region. In the ASM mode liquid crystal display device, liquid crystal molecules are axially symmetrically aligned, so that an observer recognizes less variation in contrast in any viewing directions; that is, wide viewing angle characteristics are obtained.
A production method of such an ASM mode liquid crystal display device is disclosed in Japanese Laid-Open Publication No. 7-120728. In accordance with the method disclosed in the publication, a protrusion-like structure is formed on a substrate in a grid pattern so that liquid crystal molecules are axially symmetrically aligned by the interaction between the protrusion-like structure and the liquid crystal molecules. Japanese Laid-Open Publication No. 10-133206discloses another ASM mode liquid crystal display device in which the axially symmetrical alignment is achieved by the combination of a liquid crystal material of negative dielectric anisotropy (N-type liquid crystal material) and a vertical alignment film.
A plasma addressed liquid crystal display device has potential as a large size liquid crystal display device and thus has been vigorously developed. An example of the plasma addressed liquid crystal display device is disclosed in Japanese Laid-Open Publication No. 1-217396. The plasma addressed liquid crystal display device includes a substrate, a thin dielectric sheet, ribs disposed between the substrate and the dielectric sheet, and discharge channels (plasma channel) surrounded by the substrate, the dielectric sheet and the ribs. The discharge channels are arranged in rows. The state of plasma discharge is changed by switching a voltage applied to noble gas filled in the discharge channel using an anode electrode and a cathode electrode. Liquid crystal molecules of a liquid crystal layer are driven by a voltage applied between the discharge channel and a counter electrode, via the dielectric sheet.
In the device disclosed in Japanese Laid-Open Publication No. 1-217396, liquid crystal molecules are aligned in the same direction. Therefore, the device has a problem with its viewing angle characteristics. In order to solve this problem, Japanese Laid-Open publication Nos. 9-197384 and 10-186331 each disclose a plasma addressed liquid crystal display device of the above-described ASM mode.
However, the present inventors have found that the conventional ASM mode plasma addressed liquid crystal display device has problems described in the following numbered sections (1), (2), and (3).
(1) Reduction in transmittance
In the above-described ASM mode liquid crystal display device, when the protrusion-like structure is black, i.e., light-blocking, and formed within the pixel region, an aperture ratio is significantly reduced and thus transmittance is decreased. When the protrusion-like structure is transparent, the above-described problem does not arise. Nevertheless, as schematically illustrated in FIG. 1, a liquid crystal layer 2 has a thickness of d1 and also has a thickness of d2 directly above the protrusion-like structure 1 which is smaller than d1. A portion of the liquid crystal layer having the smaller thickness of d2 does not contribute sufficiently to display. In this case, the state of display appears the same as when the transmittance is decreased due to a reduction in the aperture ratio. This occurs for the following reason. The liquid crystal display device is designed using as a reference the thickness of d1 across the region having no protrusion-like structure 1. In the case where the thickness of d2 across the liquid crystal layer 1 directly above the protrusion-like structure 1 largely differs from d1, the retardation of such a region of the liquid crystal layer is deviated from the designed value, thus reducing the amount of light contributing to display.
(2) Less stability of an axially symmetrical alignment (at a fast response speed, or under external pressure)
In the liquid crystal layer including the conventional protrusion-like structure 1, a force for regulating the alignment of liquid crystal molecules is provided by a sloped surface 1a of the protrusion-like structure 1 (see FIG. 1). Such an alignment force is hardly provided by a top surface 1b of the protrusion-like structure 1. Thus, the alignment of liquid crystal molecules is unstable in the vicinity of the top surface 1b. For example, the re-alignment of the liquid crystal molecules cannot follow a fast variation in voltage, so that the alignment state becomes irregular and thus rough display is observed.
When an external pressure is applied to a local portion of the liquid crystal cell, the less alignment force provided by the top surface 1b of the protrusion-like structure 1 cannot prevent the disturbed alignment of the liquid crystal molecules, so that the rough display is observed.
(3) Slow response speed
In a conventional plasma addressed liquid crystal display device, a voltage is applied across a liquid crystal layer and a thin dielectric sheet (e.g., a glass sheet about 50 .mu.m thick). Therefore, the voltage applied across the liquid crystal layer largely depends on the thickness of the liquid crystal layer. When the plasma addressed liquid crystal display device incorporates the ASM mode provided by the above-described protrusion-like structure, a voltage applied across the liquid crystal layer directly above the protrusion-like structure is not sufficient because such a portion of the liquid crystal layer has a smaller thickness. Therefore, the liquid crystal layer directly above the protrusion-like structure has a significantly slow response speed, reducing the entire response speed in displaying a gray scale image.
Further, since almost no alignment forces are provided by the top surface of the protrusion-like structure as described above, the alignment of the liquid crystal molecules slowly changes from the start of application of a voltage, resulting in a slow response speed. This phenomenon will be described below with reference to FIGS. 2A through 2D.
In the absence of an applied voltage as shown in FIG. 2A, liquid crystal molecules 3 are aligned substantially perpendicularly to all surfaces of a pair of substrates and a protrusion-like structure 1 facing a liquid crystal layer 4. In FIGS. 2A through 2D, a vertical alignment film is not shown for simplicity. When a voltage is applied across the liquid crystal layer 4, the liquid crystal molecules 3 first start to tilt in random directions as shown in FIG. 2B. In this case, the liquid crystal molecules 3 in the vicinity of a side wall 1a of the protrusion-like structure 1 tilt toward the inside of a region 4a (subpixel region) substantially surrounded by the protrusion-like structure 1 by an influence of the side wall 1a (see hatched portions in FIG. 2B). Then, the liquid crystal molecules 3 within the region 4a interact with the liquid crystal molecules 3 in the vicinity of the side wall 1a, and finally take an axially symmetrical alignment which is stable with respect to energy, as shown by a hatched portion in FIG. 2C. Thereafter, as shown by a hatched portion in FIG. 2D, the liquid crystal molecules 3 directly above a top surface 1b of the protrusion-like structure 1 interact with the axially symmetrically aligned liquid crystal molecules 3 within the region 4a, and thus change their alignment directions. The liquid crystal molecules 3 directly above the top surface 1b of the protrusion-like structure 1 are equally affected by the liquid crystal molecules 3 in the regions 4a surrounding the protrusion-like structure 1, and therefore are axially symmetrically aligned.
As described above, the liquid crystal molecules 3 are gradually aligned in a stepwise way, so that the response speed of the whole pixel region is slow. Also, as described above, the alignment force provided directly above the top surface 1b of the protrusion-like structure 1 is so weak that external pressure on a display panel may easily disturb the alignment, and it may take longer time to restore the disturbed alignment as compared with the liquid crystal molecules 3 in the region 4a.
Japanese Laid-Open Publication No. 7-120728 discloses a technique for stabilizing the alignment of liquid crystal molecules with a monomer to prevent the above-described phenomenon. The alignment stabilizing technique is performed in the following way: a mixture of a liquid crystal material and a photo curable monomer is injected into a cell including a pair of substrates, one of which includes a protrusion-like structure; an axially symmetrical alignment state of liquid crystal molecules is established in the presence of an applied voltage; and the monomer in the mixture is cured by ultraviolet light irradiation to form an alignment stabilizing layer which stabilizes the axially symmetrical alignment state of the liquid crystal molecules. A smaller amount of the photocurable monomer can further maintain the vertical alignment in the absence of an applied voltage. However, the use of this method leads to an increase in cost since the formation of the alignment stabilizing layer is an extra step.